This test is fundamentally racy. It opens a cursor, turns down the cursor timeout to 10 seconds, sleeps 6 seconds, then verifies that the cursor still exists. If it takes more than 10 seconds for the test to reach the check that the cursor still exists, mongos could have already timed-out and reaped the cursor.
Instead, the test should turn down the cursor timeout to a small value (1 second should do), and then use assert.soon() to poll mongos until the cursor gets reaped. This is a much less flaky way to perform the intended test that mongos eventually times out cursors.